1. Field of the Invention
This invention relates to a multilayer metallization system for silicon integrated circuits. This invention more particularly relates to an improved multilayer metallization system that includes an aluminum-silicon alloy.
2. Description of the Prior Art
Aluminum or aluminum alloys are widely used as a single layer metallization for many silicon semiconductor devices, especially integrated circuits. However, an aluminum-containing single layer metallization limits use of a silicon semiconductor device to low and moderate temperature processing conditions and product applications. Higher temperatures cannot be used because aluminum and silicon interdiffuse at elevated temperatures. On extended exposure to higher temperatures, aluminum-silicon interdiffusion and/or silicon precipitation can cause serious degradation of an aluminum-containing metallization layer on a silicon integrated circuit (IC). Unanticipated contact resistances can result. The unanticipated resistances may not allow the associated circuitry to function as designed. It may not even function at all. In any event the problem and its effects are well known, and need not be discussed further here.
Perepezko et al., in their U.S. Pat. Nos. 4,350,994 and 4,494,136, expressly recognize the aluminum/silicon interdiffusion problem. They disclose substituting another metal for aluminum in the metallization pattern. More specifically, they disclose replacing aluminum with a layer of an amorphous refractory conductive material. The amorphous refractory conductive material can be a metal, or a compound or alloy of high-temperature metals and/or metalloids.
Perepezko et al. disclose that the amorphous materials they contemplate do not interdiffuse with silicon, so long as the amorphous materials remain in their amorphous state. Hence, the amorphous state of the material acts as a diffusion barrier to silicon.
Perepezko et al. teach that an amorphous material will transform into a crystalline material upon being heated. The particular temperature at which this occurs is referred to herein as the crystallization temperature. Perepezko et al. also teach that crystallization temperature of refractory materials is higher, and highest for eutectics of refractory materials. Hence, eutectic refractory compositions are preferred.
Perepezko et al. point out that the search for appropriate metallization materials is largely empirical because there is no way to predict, a priori, whether a given metal will form a good quality ohmic or Schottky contact with a given semiconductor.
We agree. In fact, amorphous refractory materials, alone, do not make commercially satisfactory metallization layers. Our experience has shown that their specific conductivity is too low. In U.S. Pat. No. 4,965,656, Koubuchi et al. solve this problem by also using an aluminum layer, over the amorphous layer.
In other words, Koubuchi et al. interpose Perepezko et al.'s amorphous barrier layer between the silicon IC surface and the aluminum metallization layer. Koubuchi et al., like Perepezko et al., are intended to prevent silicon/aluminum interdiffusion. They both describe a wide variety of amorphous refractory metal/metalloids that can might be useful, and that eutectic compositions are preferred.
More specifically, Koubuchi et also state that the aluminum/silicon interdiffusion is a processing problem, not an operational problem. Koubuchi et al. state that detrimental interdiffusion can occur due to heating during fabrication steps following device metallization. We are aware of this problem. However, we recognize a different problem. It occurs if the aluminum metallization layer contains silicon, and it frequently does.
A frequently used aluminum metallization alloy contains 1% silicon and 1% copper, by weight and the balance aluminum. This relatively minor proportion of silicon is used to prevent "spiking" of the aluminum on a silicon IC surface. However, it is also enough to cause the "different" problem referred to above. With such aluminum/silicon alloys, post-metallization heat treatments can cause high resistance silicon precipitate formations to preferentially deposit in the metallization layer in the contact window. These preferential formations are caused by segregation of silicon atoms in the aluminum alloy. These preferential formations also occur in the metallization layer at steps in an underlying dielectric coating. Hence, silicon precipitation is not an interdiffusion problem.
In the past, the high resistance silicon precipitate formations did not pose a serious problem. This is because IC metallization contact sizes were greater than about 1.5 microns. This is fairly large compared to the size of the silicon precipitate formations. In such larger contact size ICs, the silicon precipitate formations occupied only a minor proportion of any given contact area. With IC metallization feature sizes of about 1 micron or less, we find such precipitate formations can occupy a sizable proportion of a contact area. The proportion is enough to raise contact resistance to an objectionable level. Such an occurrence at even one contact area in an IC can cause the IC to no longer perform as intended.
One might think that the solution is to simply reduce or eliminate silicon content of the aluminum alloy. The answer is not that simple. About 1% silicon is desired in an aluminum metallization alloy to avoid spiking when used on a silicon surface. It may also be included to preclude unintended interactions with other substances contacted on the silicon surface. Also, commercial manufacturers may want to use a silicon containing alloy in fine geometry applications for practical reasons. They may have large and small feature size applications and may want to use only one alloy in their facility, and/or they find comfort in using an alloy that is familiar to them. Some manufacturers may even perceive incidental other benefits in using silicon in the aluminum alloy. For such commercial manufacturers, deleting the silicon content from the metallization aluminum alloy is not a preferred choice. In this invention, we firstly solve the aluminum/silicon interdiffusion problem with a particularly effective diffusion barrier. The barrier is amorphous tungsten/silicon. In addition, silicon precipitates do not preferentially form in the contact windows when such a layer is deposited on an amorphous tungsten/silicon layer. Hence, use of the amorphous tungsten silicon also solves the silicon precipitate formation problem. Neither Perepezko et al. nor Koubuchi et al. teach that amorphous tungsten/silicon is special for either purpose in an aluminum metallization system.
On the other hand, our particularly effective amorphous tungsten/silicon material has a problem of its own. It has contact resistance problems with P-type silicon. Amorphous tungsten/silicon has good adhesion to silicon. Accordingly, it should be expected to have low contact resistance with silicon. However, in this invention, we discovered that amorphous tungsten/silicon does not consistently make good contacts to P-type silicon. Silicon integrated circuits usually have both P-type and N-type regions that must be contacted by the metallization. If low resistance contacts cannot be predictably made to all of these regions consistently, the metallization system is unsatisfactory. This invention solves the contact resistance problem of amorphous tungsten/silicon on P-type silicon by interposing an additional layer under the amorphous tungsten/silicon. Our additional layer is thus important to effective utilization of amorphous tungsten/silicon, and will hereinafter be described.
Our invention therefore provides a means for commercially using a highly effective aluminum/silicon diffusion barrier in an aluminum-containing metallization system. Thus, one preserves the option to use of an aluminum-type of system, which is well understood. Still further, our diffusion barrier material also suppresses formation of the objectionable silicon precipitation on aluminum alloy containing silicon. Thus, one can still use the familiar metallization aluminum alloy that contain silicon in current small feature size applications. In addition, an important ancillary benefit results from using an amorphous tungsten/silicon layer as the barrier layer. It is so effective as a barrier layer that it can be used in extremely thin films, which can reduce the overall thickness of the metallization layer. Minimizing the overall thickness of a metallization layer reduces step-height at the edges of the layer. This is important in obtaining most effective coverage of subsequently applied coatings, especially if such coatings are part of a multilevel metallization.